Roadway repair is often accomplished by overlaying the existing pavement (whether of concrete or asphalt paving material) with a new layer (often called a leveling course) of concrete or asphalt paving material. Without prior surface treatment, however, this method of repair generally results in the application of insufficient quantities of paving material in the rutted, potholed or otherwise damaged areas, because the overlay will be applied at the same rate per unit of roadway width in damaged areas (which have a greater depth to be filled across the width) as in the undamaged areas. The resulting reduced thickness in the overlay of the previously damaged areas will lead to renewed rutting or other wear damage in the new pavement in relatively short order. However, by milling the surface of the damaged pavement to a uniform surface elevation below the level of the damage, the addition of new pavement will produce a road surface having a consistent elevation across the entire width of the roadway. This repaving technique can be used to return the elevation of a damaged roadway to its original pre-damaged elevation, whereas the placement of a leveling course atop damaged but un-milled pavement will tend to raise the surface of the roadway or some portion thereof above its original elevation. Roadway repair without milling can require the raising of road shoulders, guardrails and manhole covers and the adjustment of overpass clearances, which tasks are unnecessary if a proper milling technique is employed. A use of milling prior to repaving can also permit ready establishment of the proper road grade and slope, and thereby avoid drainage and safety problems. Furthermore, milling typically provides a rough surface that readily accepts and bonds with the new asphalt or other pavement overlay. Finally, milling can provide raw material that can be reclaimed for use in the production of new paving materials.
A milling machine includes a milling drum with a plurality of cutter teeth mounted thereon which is contained within a milling drum housing. The milling machine is adapted to be advanced across a roadway surface while the milling drum is rotated within the housing to “mill” the surface to remove asphaltic or Portland cement concrete pavement in preparation for recycling the pavement and/or in preparation for applying a pavement overlay. The typical milling machine includes one or more conveyors to take the milled paving material from the vicinity of the milling drum and direct it away from the machine and into an adjacent dump truck. A road stabilizer/reclaimer machine is similar to a milling machine in that it comprises a wheeled or track-driven vehicle that includes a milling drum with a plurality of cutter teeth mounted thereon which is contained within a milling drum housing. However, the milling drum of a road stabilizer/reclaimer machine is generally employed to mill or pulverized an existing road bed or roadway to a greater depth than does a milling machine prior to repaving (usually called reclaiming) or prior to initial paving (usually called stabilizing), and it leaves the pulverized material in place.
Cold in-place recycling (“CIR”) equipment can be used to repair damage to a roadway in a single pass, while reusing essentially all of the existing milled paving material. In the CIR process, a milling machine is employed to remove damaged layers of pavement, and the removed paving material is processed, replaced on the roadway and compacted. If a roadway has good structural strength, CIR can be an effective treatment for all types of cracking, ruts and holes in pavement. CIR can be used to repair roadways damaged by fatigue (alligator) cracking, bleeding (of excess asphalt cement), block cracking, corrugation and shoving, joint reflective cracking, longitudinal cracking, patching, polished aggregate, potholes, raveling, rutting, slippage cracking, stripping and transverse (thermal) cracking. The root cause of the pavement failure should always be investigated to rule out base failure. However, CIR can almost always be used when there is no damage to the base of the roadway. Generally, CIR is only half as expensive as hot mix paving (i.e., paving with new asphalt paving material) while providing approximately 80% of the strength of hot mix paving.
CIR can be carried out with the aid of a milling machine or a road stabilizer/reclaimer machine that has been modified by mounting an additive spray bar in the milling drum housing to inject asphalt cement into the milling drum housing. The asphalt cement is then thoroughly blended with the milled material by the milling drum. This blended mixture may then be deposited on the roadway in the form of a windrow to be picked up by a suitably equipped asphalt paving machine, or it may be fed by the milling machine's discharge conveyor directly into the receiving hopper of an asphalt paving machine. When the CIR process is carried out with only a milling machine or stabilizer/reclaimer and an asphalt paving machine, the asphalt cement component of the mixture must be supplied from a separate asphalt cement supply tank carried by a supply truck, vehicle, wheeled chassis or other mobile support that is typically coupled to the modified milling machine or road stabilizer/reclaimer machine. The asphalt cement component is drawn directly from the tank on the supply truck and metered through an asphalt cement flow circuit to the spray bar in the milling drum housing.
Sometimes the CIR process is carried out with a milling machine or stabilizer/reclaimer in train with a cold recycler machine such as the RT-500 that is made and sold by Roadtec, Inc. of Chattanooga, Tenn. The cold recycler machine may include a vibratory screen, a crusher, an onboard source of asphalt cement and a pugmill mixer. When the CIR process is carried out using a cold recycler machine, the recycled paving material that is milled by the milling machine is transferred to the vibratory screen. Oversized material on the screen is conveyed into a crusher on the cold recycler machine, and material passing through the crusher is carried back to the screen by means of a return circuit. Properly sized material is then mixed with asphalt cement from an onboard storage tank in the pugmill mixer. Because the onboard asphalt cement storage tank in a cold recycler machine is limited in size, it may be desirable to convey additional asphalt cement from a separate supply truck to the asphalt cement storage tank on the cold recycler machine, in order to insure that the CIR process can proceed without frequent stops to refill the asphalt cement storage tank on the cold recycler machine. In either configuration of the CIR equipment, the primary component of the new pavement is paving material that is removed from the roadway. The only other component of the new pavement is the asphalt cement carried by the cold recycler machine and/or by a mobile supply vehicle. Since the rate of advance of the equipment engaged in the CIR process is determined primarily by the rate of advance of the milling machine, it is common for all of the components of the CIR process except for the asphalt paving machine to be coupled together so as to move at the same rate during all phases of the CIR process. Such components employed in carrying out a CIR process are frequently referred to as a CIR train.
Asphalt cement performs best in the CIR process when it is applied at a temperature within the range of about 300° to about 350° F. Although the asphalt cement supply truck is typically thermally insulated, it does not include any heating mechanism for maintaining the temperature of the asphalt cement as the CIR process is carried out. Consequently, the asphalt cement in the supply truck will begin to lose heat as soon as the truck leaves the asphalt cement supply terminal. If the CIR process is being carried out at a great distance from the asphalt cement supply terminal, the asphalt cement in the supply truck will have lost a significant part of its heat even before the CIR process is begun. Furthermore, the CIR process may begin at a time of day such that it cannot be completed during a single operating shift. When the temperature of the asphalt cement in the supply truck falls below about 300° F., for whatever reason, its continued use will likely result in a repaired roadway of substandard quality.
Co-pending U.S. patent application Ser. No. 15/855,403 describes a method and apparatus for heating asphalt cement that is carried in an asphalt cement supply tank prior to the use of such asphalt cement in a CIR process. The apparatus described in this application comprises a heater for asphalt cement that is a part of a CIR train. More particularly, the heater is interposed between the asphalt cement supply tank and the component of the CIR train that is dispensing asphalt cement to be mixed with recycled paving material in the CIR process. Preferably, a heater with an infinitely variable heating output between about 300,000 BTUs per hour and about 500,000-750,000 BTUs per hour is employed in the method of this co-pending patent application; however, infinitely variable heaters with heating outputs in this range are difficult to find. Consequently, it would be desirable if an apparatus could be provided that would allow for heating asphalt cement by the required amount using a much simpler heating apparatus.